addDaysToDate

Calculates the date of n days from the given date, returning its string representation.

• Use new Date() to create a date object from the first argument.
• Use Date.prototype.getDate() and Date.prototype.setDate() to add n days to the given date.
• Use Date.prototype.toISOString() to return a string in yyyy-mm-dd format.

const addDaysToDate = (date, n) => {
  const d = new Date(date);
  d.setDate(d.getDate() + n);
  return d.toISOString().split('T')[0];
};

addDaysToDate('2020-10-15', 10); // '2020-10-25'
addDaysToDate('2020-10-15', -10); // '2020-10-05'